1. Field of the Invention
The present invention relates to a semiconductor storage device and a method for manufacturing the same, and more particularly to a semiconductor storage device which has a trench structure and a method for manufacturing the same.
2. Description of the Related Art
Miniaturization of a semiconductor device has been accompanied by a reduction in an area of a capacitor which stores charges in a dynamic random access memory (DRAM), and it is now a significant task to secure a sufficient capacitance.
Thinning of a capacitor insulator film is effective for increasing capacitance of the capacitor with the same area. However, it poses a problem of an increase in leakage current. When a high dielectric-constant insulator is used as a capacitor insulator, the capacitor can be increased in capacitance without any increase in leakage current using an insulator with the same thickness. Thus, studies have been conducted to change the capacitor insulator from a silicon oxy-nitride (SiNO) film commonly used at present to a high dielectric-constant insulator, e.g., an aluminum oxide (Al2O3) film.
As another method for increasing the capacitance of the capacitor, increasing an effective surface area of a capacitor is disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2001-200363. According to this method, a silicon film having a rugged surface, e.g., a hemi-spherical grain (HSG) surface, is used as a lower electrode of a stack or trench capacitor. Generally, formation of a high-quality insulator on such a rugged surface in a sufficient coverage is not easy. A method in that patent overcomes the problem to increase the coverage better. However, a process thereof is complex, and more improvements need to use it practically. Additionally, the trench capacitor uses a trench inner wall as a capacitor. Thus, a trench diameter must be increased to use the HSG silicon film, which is not beneficial for promoting miniaturization.
As another problem, it has been discovered that, for example, when polysilicon is used as a capacitor electrode, a depletion layer formed in the electrode substantially lowers the capacitance of the capacitor to reduce effects of the thinning of the capacitor insulator. The formation of the depletion layer in the electrode can be suppressed by setting a high dopant concentration in polysilicon or silicon used for the electrode.
In a present trench DRAM capacitor, doping to one of electrode (plate electrode) formed in a silicon substrate is carried out by, e.g., solid-phase diffusion from arseno-silicate glass (AsSG) or gas phase diffusion using phosphine (PH3). For example, when a doping amount of arsenic (As) in the plate electrode is set high, As is also doped into silicon substrate in an upper part of the trench by auto-doping, where the doping of As is undesired. Consequently, even a channel region of a metal oxide semiconductor (MOS) transistor is doped with As, causing a problem of deterioration of subthreshold characteristics.
Therefore, there is a need for a semiconductor storage device comprising a trench capacitor in which a high dielectric-constant insulator is used and formation of a depletion layer in a capacitor electrode is suppressed, and a method for manufacturing the same.